There has been conventionally known a differential gear used in a vehicle such as an automobile, which transmits a driving force (torque) from an engine, through a ring gear, a differential case installed on the ring gear, a pinion shaft installed on the differential case, pinion gears installed on the pinion shaft, and side gears configured to be meshed with the pinion gears. The pinion shaft, the pinion gears, and the side gears, are arranged inside the differential case.
As a method of forming such a differential case of the differential gear, there is known a technique including the steps of: preparing a first differential case and a second differential case; integrally molding the first differential case with a ring gear by forging; integrally molding the second differential case with a pinion-shaft installation part on which a pinion shaft is installed, by forging; and assembling the first differential case and the second differential case by a plurality of bolts (see, JP2000-266162-A).
In another method of forming such a differential case of the differential gear, a first differential case and a second differential case are firstly prepared. The first differential case is molded by pressing, and the second differential case having a pinion-shaft installation part is molded by cold rolling. Then, a ring gear is separately molded. Thereafter, the first differential case and the second differential case are bonded by welding, and the second differential case and the ring gear are bonded by welding (see, JP2006-509172-A).